This invention relates to a process for producing ethanol from methanol, carbon monoxide and hydrogen selectively.
It was known in the prior art that ethanol was produced from methanol, carbon monoxide and hydrogen by using a catalyst comprising a cobalt component and an iodine or bromine component and optionally a ruthenium component, an osmium component and a ligand. For example, Japanese Patent Publication (kokoku) No. 24863/1963 discloses a process for producing ethanol which comprises reacting methanol, carbon monoxide and hydrogen in the presence of a cobalt catalyst and an iodine co-catalyst. U.S. Pat. No. 3,285,948 discloses a process for producing ethanol which comprises reacting methanol, carbon monoxide and hydrogen in the presence of a catalyst comprising a soluble cobalt compound, iodine or an iodine compound and a ruthenium compound.
Japanese Patent Publication (kokai) No. 147009/1978 discloses a process for producing ethanol which comprises reacting methanol, carbon monoxide and hydrogen in the presence of a cobalt catalyst, an iodine or bromine and an organic phosphorus compound co-catalyst and a large amount of monocarboxylic acid or derivatives thereof. However, such monocarboxylic acid or derivatives thereof forms an ester with the ethanol produced by the reaction or the methanol which is a starting material; and the use of large amount of such acid lowers the selectivity to neat ethanol. Moreover, the resulting complex composition of the reaction mixture makes the recovering process for ethanol complicated.
Recently, it has been proposed to add a variety of ligands, such as tertiary phosphine, tertiary arsines or tertiary antimony to the prior catalyst for producing ethanol from methanol, carbon monoxide and hydrogen. For example, British Pat. No. 1,546,428 discloses a process for producing ethanol by reacting methanol, carbon monoxide and hydrogen in a hydrocarbon solvent in the presence of the catalyst composed of cobalt-halide-tertiary phosphine.
Japanese Patent Publication (kokai) No. 49326/1980 discloses a multidentate ligand containing cobalt-bromine or iodine-nitrogen or phosphorus atom as such catalyst.
British Pat. No. 2,036,739 discloses a process for producing ethanol by reacting methanol, carbon monoxide and hydrogen in the presence of a catalyst composed of cobalt and a metal of Group VIII of the Periodic Table (Fe, Ru, Os, Rh, Ir, Ni, Pd and Pt), promoter composed of a tertiary phosphine, a tertiary arsine or a tertiary antimony and iodide or bromine.
Japanese Patent Publication (kokai) No. 92330/1980 discloses a catalyst comprising hydride cobalt carbonyl complex, iodine, a ruthenium compound and a tertiary phosphine, a tertiary antimony or a tertiary arsine as such catalyst. U.S. Pat. No. 4,233,466 discloses a catalyst for production of ethanol from methanol, carbon monoxide and hydrogen which comprises cobalt, ruthenium, iodine and a tertiary phosphine in which a molar ratio of phosphorus to iodine is between 1:0.36-1:5 and a molar ratio of phosphorus to cobalt is more than 1.5.
However, when methanol is reacted with carbon monoxide and hydrogen in the presence of any one of these known catalysts, by-products, such as dimethyl ether, methyl ethyl ether, diethyl ether, acetaldehyde, dimethoxy ethane, acetic acid, methyl formate, methyl acetate, ethyl acetate and other compounds of C.sub.2 or more other than ethanol (object product) are formed, so selectivity to neat ethanol is low.
On the other hand, Japanese Patent Publication (kokoku) No. 24863/1963 or U.S. Pat. No. 3,285,948 discloses a process for producing ethanol which comprises reacting methanol, carbon monoxide and hydrogen at a temperature of 175.degree.-230.degree. C. and a pressure of more than 281 Kg/cm.sup.2 G in the presence of a catalyst comprising cobalt-iodine or a catalyst comprising cobalt-iodine-ruthenium and in the absence of any solvent. Since the catalysts do not contain any ligands, it is easy to handle them. However, when the above catalysts are used, by-products, such as ethers and methyl acetate are formed in a large amount, so selectivity to neat ethanol becomes lower extremely.
When a catalysts comprising cobalt-iodine or cobalt-iodine-ruthenium and a ligand are used, formation of by-product, such as ethers are suppressed. However, since the catalysts contain ligands, activity of the catalysts is lowered. Therefore, in case of using catalysts containing ligands, the reaction must be carried out at a higher temperature. This results in forming by-products and lowering selectivity to ethanol. Particularly, the catalysts contain cobalt and ruthenium as well as iodine or bromine and ligands, such as a tertiary phosphine, a tertiary antimony or a tertiary arsine as a ligand, so the following shortcomings are produced in case of carrying out the reaction in the presence of the catalyst on a commercial base:
Since ligands, such as a tertiary phosphine are instable to heat, the ligands are likely to be decomposed in the reaction system, or quality of the ligands is likely to be changed. Therefore, it is difficult to recover the active catalysts from the system. In addition, recovery of each component constituting the catalysts not only need complicated operation, but also loss of the catalysts is great in recovery of the catalysts. Since the catalysts are costly, loss of the catalysts is unpreferablly in case of recovering the catalysts.
In the prior processes, there were problems in respect of selectivity to ethanol, reaction speed and recovery of the catalyst. The prior processes are not industrially satisfactory.